Control system for a variable pitch propeller



CONTROL SYSTEM FOR A VARIABLE PITCH PROPELLER Filed oct. 28. 1955 Jne24, 1958 c. M. .JEDRZlEwsKl ErAL 2 Sheets-Sheet 1 NM MS w98 mm Dm NQQ.Ee 24, 1958 c. M. JEDRzIEwsKl Erm. 2,840,171 l CONTROL SYSTEM FOR AVARIABLE FITCH PROPELLER 2 Sheets--Sheeil 2 FiledV Oct. 28, 1955ATTORNEYS Y 2,840,171 Patented June 24, 1958Anw CONTROL SYSTEM FOR AVARIABL PITCH PROPELLER Y Chester M. Jedrziewski, Rockville, and DonaldP.

Ramaker, Glastonbury, Conn., assignors to United Aircraft Corporation,East Hartford, Conn., a corporation of Delaware Application October 2S,19755, Serial No. 543,465 13 Claims. (Cl. 170-160.2)

This invention relates to aircraft propellers and, more particularly, toanimproved` propeller pitch control and servo system of the general typeshown and described in the co-pending application of Stanley G. Best,Serial No. 426,061, filed April 28, 1954. Y

it is the general object of the invention to provide a control systemfor the pitch changing mechanism of an aeronautical propeller and whichincludes a lsource of hydraulic fluid under pressure for operatingy aservo motor to actuate the said pitch changing mechanism, and it is amore specic object of the invention to provide means for controlling theflow of hydraulic lluid to the servo motor in such a way that the bladeangle may be decreased at a lesser rate than it is increased whereby toreduce the likelihood or possibility of propeller overspeeding duringnormalV pitch changes and when the propeller is moved from the featheredposition into' the normal operating range. In addition to the foregoing,it is an object of the invention to provide the means for controllingpitch changing rate in the afore-described manner and-which will notVoperate to limit the rate of reduction when theV blade vangle is beingreversed.

As the description of the invention proceeds, it will4 become apparentthat the control and servo system of this invention isnot limited inapplication to any'particular propeller or to any particular pitchchanging mechanism. Therefore, the drawings do not include a disclosureof a propeller assembly or of a pitch 'change mechanism. U. VS. Patent2,652,122 to Longfellow may -be relied uponfor a disclosure of anexemplary `propeller assembly and pitch'change mechanism VWhichvcan beoperated by the pitch control and servo system'of 'thisinvention. f Ofthe drawings:

Fig. 1 is a schematic illustrationof an hydraulic pitch control andservo system for an aeronautical propeller assembly and whichincorporates the features of the present invention; and y Fig. 2schematically illustrates the invention in a modified or alternativehydraulic control and servo system.

In its broadest aspects, the system of this invention may be said toinclude pump means for supplying the system with hydraulic iluid at asubstantially constant pressurev which uid is directed to a reversibleservo motor to drive the propeller pitch changing mechanism. The systemalso includes a pair of propeller speed responsive llow control valveswhich introduce the hydraulic fluid to the servo motor as required tocauseoperation thereof toeffect a necessary pitch change. The systemalso includes valve means which are operable to controlV iluid flow tothe flow control valves and/or Vthe servo motor during extraordinarypropeller pitch changes as, for example, when the propeller is featheredand unfeathered and when its pitch is reversed and'returned to thepositive pitch range. As a particular Vfeature r4of this invention,means are provided for reducing thejrate of movement of the servo motor,and thus the rate of the pitch change, in the low pitch direction ascompared to the rate in the height ered angle into the normal operatingrange. The invention is also featured by means for providing arelatively rapid rate of pitch change in the low pitch direction whenthe propeller blade angle is being moved into thereverseV pitch range..l Y in describing the invention in greater detail and with specificreference to the drawings, it will be observed that the means providingthe system with hydraulic fluid under-substantially constant pressurecomprise a main pump unit 10 which includes one pump 14 adapted towithdraw oil or other hydraulic iluid from an atmospheric sump l2 and todischarge the same into a pressurized sump 15. Another pump 16, includedin the main pump unit, withdraws the oil from vthe pressurized sump 15and discharge the same into the system through a conduit 18. 'Y

It is preferred that the aforementioned main pump unit 1) comprise apairof gear pumps 14 and 16 arranged in tandem fashion and connected to agear train 19 driven bya gear 2li which rotates with the propeller. Asshown in the aforementioned Longfellow patent, the drive gear 2t) maycomprise a large diameter gear forming a part of the hub section of thepropeller so as to rotate at equal speed therewith. Accordingly, duringnormal pro peller` operation, the tandem pump unit 10 and the sumps 12and 15 provide a source and means for continuously introducing hydraulicuid under Apressure to the control system.

A generally similar tandem pump unit 21, including the pumps 22'and 23,is adapted to supply the system during the periods that the propeller isnot rotated. MoreI specifically, the pump unit 2.1 is driven through agear train 24 by an electric motor 25 Whichrmay be manually controlledfor operation whileY the propeller is feathered or during very low speedengine operation. While not a part of this invention, it will be readilyunderstood that automatically operable means may be utilized to energizeandv de-energize the auxiliary motor 25 at preselected propelleroperating speeds or positions. y y

The hydraulic iluid which is introduced to the system through the line18 passes first through a check valve 27 to maintain pressure within thesystem. *Having passed the check valve 27, the hydraulic fluid passesinto two main intercommunicating branch lines Vor passages; 28 and 29. Ahigh pressure relief valve 30 is connected inthe line28 vto fix theupper limit' of pressure withinthesystem at a predetermined level, thevalve30 being 'con-Y nected to the pressurized sump l5 to drainthereinto in the event the selected pressure isr exceeded withiny thesystem. The line 28 terminates within a servo cylinder 32 which,as willbe more fully explained hereinafter,Y constitutes anV important part ofthe vservo motor and which transmits the motion and force to'operatetheY propeller pitch changing mechanism (not shown).

The main branch line 29 extends into a normally open valve 33 which whenclosed shuts olf a portion of thev system Vand conditions the same forfeathering the pro` peller. A passage or line 34l extending from thevalve; 33, and normally in communication with the main branch line 29,extends to a solenoid operated valve 35 which, as will be explainedhereinafter, is utilized to condition the system for reversing the pitchof the propeller. l

The various valves included in the systemv are shown in ing at theequilibrium or Yselected Vspeed'forra 'power' setting selected andfxedybythe pilot. Therefore, it may be said that the valves as shown reflectan on speedv propeller operating condition. The' particular angleof` theblades is reected in the position of the `servo cylinder:

32 which" is arranged foraxial reciprocation to increase and decreasethe pitch or blade angle. More specifically,

by thearrow,` causes a "change/toward low pitch, it willV beseen thatrighthand movement of th'e cylinder is accompanied by movement rof thepropeller towards low pitch and that left-hand movement of the cylinderis accompanied by propeller movement. towards high pitch.

As` shownin the drawing, the cylinder 32 is arranged to reciprocate overa fixed piston 40. The hydraulic tluid under pump pressure in the line28 is introduced to the cylinder in the chamber 41 on the left-hand sideof the piston 40;' The cylinder 32' is exposed over Vsubstantially lessarea to the chamber 41 than to the chamber 42 defined 'atth'e other endof the cylinder. For exemplary purposes, it'may be assumed that theexposed left end face area ofthe cylinder is 1/z the exposed rightendface area thereof. Accordingly, it will .be seen that fluid within thechamber 4 2 under half the pressure of the uid within thecharnber 41(half pump pressure) will effect a pressure balance on the opposite endsof the cylinder `32 so that no movement of the said cylinder will beeffected; It will be readily understood that cylinder reciprocation canbe brought about by uid flow into and out of one or both of the chambers41 and 42. lthas beenfound mostconvenient to causefreciprocation of thecylinder 32 by fluid flow into and out of the chamber 42. Fluid ow intothe chamber 42 will,'ofc'ourse, increase the'tluid pressure thereinVrwhile flow from the chamber 42 will decrease the pressure therein. IfVthe increase in uid 'orservo pressure (Ps) in chamber 42 is in excessof the operating load and forces imposed on the cylinder 32, the saidcylinder will be moved toward 'the right to cause propeller change inthe low pitch direction. If the decrease in servo pressure (Ps) inchamber 42 is in excess ofthe 4operatirig'load and forces imposed on thecylinder`32, the said cylindervwill be moved toward the' left to causepropeller change in the high pitch direction. As set forth `in theaforesaid Best application, means are provided for producing uidpressure a xed amount greater than pressure (Ps). within the chamber 42and other means are provided tor produce fluid pressure Ya fixed amountless Athanls pressure, the produced pressures being used for servovmotor operation.- The said pressurelproducingvmeans `comprise a pair of'pressure regulating valves indicated generally by the reference`numerals V43 and 44. The pressure regulating valves 43and44areconnected to two Aflow control-valves 45 and 46, th'ef valve 46 beingadaptedgduring normal operation to `selectively connect the pressureregulating valves 43 a'n'd 44 with the` servo chamber 42, and theA valve4 5 being included as Va safetyhflow control valve to preventpropellerioverspeeding. i A Y Y With 'specific reference'to the valve43, it will be noted thatthe vsar-ne includes an 'axially movablen valve.member 47 which is biased `in one` direction by `a spring48. `Thechamber. 49. surrounding `the said spring communicates with the servochamber 42 through the passage 50 which extends .through rthe .piston40.,` Accordingly, the tluid pressurelwithin the .spring chamber 49 isequal to-the fluid pressurelPs) ,in ,the servoV chamber V42 and exertspressureta'gainstthe bottom of thevalve member47 which pressurecissupplemented by the force `of the spring` 48. The iiuidpressureyandspringV force tend to move the valve member 41 upwardly to closeatchamber l'51 at the top iend of` the valvexrnember.` Howevenywhen th'evalve memberl 47 isshifted upwardly, a passage `52,extendingatan-obliqa're angle :throughthe valvetnember tand-open#`ingtintotthtchamber 5'1, is 'brought into communication with-.a` passage53x1c'onnected to the` 1ine.34 having hydraulic `fluidat pump pressure.For `the purpose of valve member 47 downwardly in opposition to thecorn-- bined fluid and spring pressure in the chamber 49. When the valvemember 47 is thrust downwardly to the position shown, the internalpassageway 52 will no longer be connected to the source of hydrauliciluid at pump pressure and pressure in the chamber 51 will equal theservo pressure within the spring chamber 49 plus the pressure equivalentexerted by the. spring 48. Inother words, the chamber 51 in the valve 43provides a source of hydraulic fluid at a pressure a fixed amountgreaterthan the pressure (Ps) within the servo chamber 42. Assumingspring 48 to have a pressure equivalent of 30 p. s. i., the highpressure chamber will :always have fluid at 30 p. s. i. greater thanservo pressure. As shown in the drawing, the high pressure chamber 51 isconnected by the passage 54 to the uppermost part of the control valve45.

The pressure regulating valve 44 includes an axially movable valve body55 which is-biased downwardly by a spring 56. The chamber 57'surrounding the spring 56 is connected by a passageway 58 to thelowermost port on the valve 45 and is also connected by the branchpassageway 58a to the lowermost port on the valve 46. The chamber 59 onthe opposite side of the movable member 57 is in communication'with theservo chamber 42 through the passage 50. Accordingly, the uid pressurewithin the chamber 59 will equal the pressure (Ps) within the servochamber 42. It will be seen that the servo pressure within the chamber59 must equal the iluid pressure within the chamber 57 plus the force ofthe spring 56 which, for purpose of illustration, may be assumed toexert a force on the valve body 55 equivalent to 30 p. s. i. of fluidpressure, or the valve' member 55 Y will be thrust downwardly tending toclose the chamber r lsage 60 extends to the pressurized sump 15 whereinthe hydraulic uid is contained at considerably less than servo pressure(Ps). Thus, the liuid pressure within the chamber 57,will.be balanced intheposition shown to close the drainpassage. In such position of thevalve member, the fluid pressure within the chamber 57v and passagewayl58 will be 30 lbs. per sq. inch less than the pressure (Ps) withintheservo chamber. The pressure drop between the lower faces ofrvalvemembers 47 and 55 and the annular grooves therein-is substantiallyconstant.

As previously mentioned, the aforedescribed pressure regulating valves43 and`44 are cooperatively associated with the control valves 45 and 46which function to selectively connect fluid passagesV from the saidpressure regulating valves to the servo cylinder chamber 42. Each of thesaid control valves includes an axially movable valve memberr which issubjected to movement in re- ,serious overspeeding vof 4the propeller.

in substantiallythe position shown.

rsaid valve member will bemoved inra manner to be more Y fully describedhereinafter.

Y' The valve 46 `may properlyfbe referred to asa proportional solenoidoperated `flow-control valve and functions `toselectively control flowbetween the pressure regulating valves 43 and 44 and the servo chamber42 throughout the said preselected range of operating speeds, therebyexercising precise speed control of the propeller in keeping with aselected power setting. As will be more fully described, electronicmeans areprovided to transmit a signal to the coils of the solenoid, inresponse to propeller speed variations from the selected level, to shiftthe position of the movable member included in the valve structure. Thesaid movable member is shown in the position assumed when no signal isbeing transmitted to the coils, `as when the propeller is rotating atthe selected speed.

Before describing the flow control valves 45 and 46 in detail, it isimportant to note that the high pressure passage 54 from the valve 43communicates with the uppermost port on the valve 45 and that highpressure uid ows through a a normally open port in the valve 45 into apassage 61 which communicates with the uppermost port in the solenoidoperated distributor valve 46. It should also be noted that theintermediate ports `of the valves 45 and 46 are interconnected by theline 62 which communicates with the servo chamber 42 by means of thepassage 63. Accordingly, in top-to-bottom order the valves 45 and 46 arein communication with hydraulic fluid at pressures a ixed amount greaterthan the pressure (Ps) in the servo cylinder chamber 42, at the pressure(Ps) in the servo chamber and at a pressure a xed amount less than servochamber pressure (Ps).

With more specific reference to the topping governor valve 45, itY willbe seen that the axially movable valve member is designated by thereference numeral 65. The said valve member moves within a sleeve 66with the upper portion of the valve member extending above the sleeveand forming a part of a centrifugally operated governor mechanismindicated generally by the reference numeral 67. Included in thegovernor mechanism is a Speeder spring 68 which is seated upon a plate69 secured to the projecting portion of the valve member. The springtends to force the valve member downwardly within the sleeve to theposition shown. rthe spring force is opposed by a pair of llyweights'76, 70 which are pivotally mounted on a gear 71 to engage the bottomface of the plate 69. The gear 71vis driven by the propeller throughgears 72 and 73, it being shown that gear 72 is engaged by the propellerring gear 2t). It will be readily understood that as the propellerrotates and gear 71 is driven, the ilyweights 70, 70 tend to pivotoutwardly and thereby exert a force on the valve member 65 in oppositionto the Speeder springl 68. Accordingly, the Speeder Spring may beselected to permit upward movement -of the valve member at a preselectedpropeller speed. ln accordance with the invention, upward movement ofthe valve member 65 is utilized to stop ow of high pressure oil towardthe servo cylinder 32 so that there can be no low pitch movement of thesaid cylinder.

More specifically, the valve member 65 is provided with a land 74adapted to selectively close the ports in communication with passages 61and 62. During' normal propeller operation, i. e., While the propellerisoperating below the selected maximum speed, passage 62 (containing uidat servo chamber pressure) will be closed at the valve 45 by the land74. In this position of the valve member, high pressure flow is accommo;dated from the high pressure regulating valve 43 to the solenoidOperated control valve 46 through passage 61.

When centrifugal forces brought about b'y anoverspeeding propellerovercome the force of the Speeder spring and valve member 65 is shiftedupwardly; the land 74 will close passage 6.1 and open valve 45 to thepassage 62 containing fluid at servo chamber pressure (Ps). In the lastmentioned position of the valve member 65, fluid will tlow through valve45 from passage 62 into pass'ageway 58 extending to the low pressureregulating valve 44. This flow of uid at substantially the pressurepresent in the servo chamber 42 and valve chamber 59 will force valvebody SS'downwardly in valve 44 so that drain passageway 60 is openedtherein. Thereby,'fluid from the servo chamber is drained to thepressurized sump, and the servo pressure (Ps) is reduced to permitleft-hand or high pitch movement of the servo cylinder.

From the foregoing description, it should be fully understood that thetopping governor valve45 provides means for limiting the speed of apropeller at a fixed level which is determined'by selection of theSpeeder spring. During propeller operation at speeds below the selectedlimit, the valve 45 has no effect and permits ilow of high pressurefluid to the solenoid operated ilow control or distributor valve 46whereupon said fluid may be utilized to decrease pitch and increasepropeller speed in the exercise of exacting control of propeller speed.

With specific reference to the valve46, it will be seen that a valvemember 75 is arranged for movement within a sleeve '76 and projectsthere'above. The projecting portion of the said member comprises thearmature of a proportional solenoid 77 which may include one or morecoils adapted upon being energized by signals to move the armature andvalve member upwardly or downwardly, the magnitude of movement dependingupon the potential of the Signal. The means for transmitting the signalmay comprise an electronic governor (not shown). The said electronicgovernor is arranged to sense a propeller speed variation from the speedin keeping with a selected power setting. More specifically, when thepilot makes a setting of his power controls, the propeller should rotateat a certain or specified speed for such power setting for optimumeiciency. Changes in air density and changes in aircraft ight attitudeswill inuence propeller speed. The electronic governor is actuated eitherby a change in the propeller speed when governing or by adjustment ofthe power setting to generate and transmit the proper signal to thesolenoid 77 whereby to adjust the position of valve member 75 which, aswill be'shown, causes servo operation to increase or decrease propellerpitch for the purpose of controlling the propeller speed.

A land 78 is provided on the movable valve member 75 to close andselectively open the passage 62, which is connected to the servochamber42, for high pressure ow and for draining purposes. Morespecifically, the land 7S normally closes .the passage 62, Vas when nocorrecting signal is transmitted by the electronic governor. When thevalve member 75 is thrust downwardly by the solenoid, as will occurupona signal transmitted Vas a result of underspeeding, the passage 62 isopened for the ow of fluid from the high pressure passage 61. Theincrease of pressure in passage 62 causes increased pressure (Ps) in theservo chamber 42, resulting in a servo cylinder shift toward low pitchto increase propeller speed. When the valve member is elevated by thesolenoid in response to propeller overspeeding, passage 62 will beopened to drain tluid from the servo chamber into the passage 58a tothelow pressure regulating valve 44.

It is important to observe that ow of uid into chamber 42 increases thepressure (Ps) therein. The new or increased pressure in chamber42 isreected in the pressure in the spring chamber 49 of the high pressureregulating Valve 43. VAs previously described, an increase in pressurein the said springchamber opens the valve 43 to the ow of fluid at pumppressure until the pressure in high pressure passages 54 and 61 is 30p.s. i.

in excess of the pressure in the spring chamber 49. Ac-

cordingly, as the servo pressure (Ps) in chamber 42 n. passageV 61increases creases, the pressure in high pressure a corresponding amountandthe pressure drop in control valve 46 from passage 61 topassage 62will always be 3()l Y p.s. i. As a result, the flov7 rate to the chamber42'will not vary with pressure changes in thechamber 42 and the ratewill be xed by the opening alorded in the valve 46,' said opening beingdetermined by the-amount/of movement of/Uthe valve member 75.. as iscontrolled by the .proporzional solenoid.4 v Y #Elow `from theuchamber42 through valve 46` will; decrease the pressure (Ps) in the chamber 42`which decreasedpressure (.Ps.) is also.presentin chamber 59 of the lowpressure regulating valve 44. As previously described, a-.decrease. in.pressure in.chat11ber 59.' opens the .valve 44^.t`owdrain until.`the'pressure in' low. pressure. passages 58 and 58a is30p. s. i.below'the pressure (Psi) in servo chamber42. Accordingly, as.the servopressure (Ps) in chamber 422 decreases, the pressure inv passage 53a decreases a `corresponding amount. and the pressure drop in controll valve46-Yfrom passage/62 to passage 58a will always` be, p. s. i. As aresult, the flow rate from servo-chamber 42 willinotr'vary with pressurechanges l in chamber 42 and the rate will befixed by the valve 46 ascont-rolled by the proportional solenoid. Y

`It= will be understoodxthenrthat ow to or from theservoi chamber- 42willbefully controlled by the valve 46r whichu selectively connectstheservo chamber to a valve 43 regulating the pressure of an hydrauliclluid` sourceto a pressure a fixedy amount (3() p. s. i.) greater thanservo' pressure or to a valve 44fregulating'pressure in Y a sump=conduittoa pressure a xed amount (30 p. s. i.)

less than servo pressure: Therefore, there is a fixed con'- l" stant'pressure-dropetol or'from the' servoA chamber 42 and the valve 46controls the rate of ow to or from the servol chamber in response to thesignal transmitted to itssolenoid 77. Now, it will be quite apparentthat a signal to valve 46 calling for a correction in the low pitchdirection shouldcause flow through valve 46 to the ser-vo chamber 42 `ata `rate. equal to the rate of flow from the chamber 42 through valve 46when an equal signal is received calling for a correction in the highpitch direction. Such actually is the case in the systemas thus fardescribed and"`i`n the system shown in the aforementioned Bestapplication. However, inthe system of this invention, means are providedto limit the rate of flow from the highpressure regulating valve l 43 tothe control valve 46'thus limiting the rate at which uid will flow tothe servo chamber andthu's ,limiting or reducing the rate at which lthepitch or blade angle of the propeller can'be decreased. yTheaforementioned rate limiting means will be described in detailhereinafter. i

Itis believed that a description of the operationV of the servo 'controlsystem as thus far described .will serve to further the understandingofthe invention. Assuming that vthere is no hydraulic fluid withinfthesystem and that the` aircraft engine is not operating, it will beapparent that the auxiliary motor 25 mustV be energized to operate pumpunit l21 to supply the system vwith hydraulicY vuid.' Having passed thecheck valve 27, the hydraulic uid is introduced to-the two main branchlines 28 and 29.. VThe lluid passing through rthe line`28 will reach thechamber 41 within the servo cylinder` 32. Since there is no fluidpressure within` the Vservo chamber 42, the servo. cylinder will beshifted toward the left to increase propeller pitch. The extent of.movement of the/servoA cylinder. in the high pitch directionmay belimited by an' elongatedV stop `rod 85 which is adjustably securedwithinthe piston 40. "['he'saidstopod may be adjusted to project as.desired into ,the` servo chamber 42 so as to ,engage the plate 86'closing the right-hand end of the servo cylinderina selectedposition ofthe cylinder. ,Accordingly, the extent` of high Lpitch movement of thecylinder fmay `be limited to correspond to a propeller pitch angleilessthan a full feather position. However,

, itgis preferred that the stop. rod 85 Vbefiixed to permit a change inthe pitchangle of the. propeller toa fullA feather i position.Thus,`ffor the.purpfo'se'of` this description, it maybe assumedthatthefservo cylinder 32.can.be shifted farenough to lpertrrit.feathering of the propeller.

The hydraulic duidfwhich `enters the main branch line CII 29 passesthrough the'normally open shut ott valver33Y flow through the line 54and the topping governor valve 45 to the proportional solenoid operatedvalve 46. Since there is normally nov signal transmitted to theproportional solenoid valve during engine inoperation, the solenoidoperated valve member will assume the neutral position shown. In suchposition, there can be no now of high pressure iluid to the servocylinder 42. Therefore, in order to supply the servo chamber withhydraulic huid, it is necessary to impose a signal upon the proportionalsolenoid 77 to thrust the valve member 75 downwardly. lt may bedesirable to have a manually operable signal generator connected to theproportional solenoid tor purposes of ground operation or means may beutilized to generate ay signall within the electronic governor, whichmeans should be adapted to selectively energize the electronic governorsimilarly to an overspeeding or underspeeding propeller. When the valvemember '75 is thrust downwardly, hydraulic lluid at pump pressure willnew from the high pressure passage 61 through the `valve 46 into thepassages 62 and 63 and thus to the servo chamber 42. Upon receivinghydraulic fluid within the servo chamber 42, the servo cylinder 32 willbe moved toward the right or in the low pitch direction. Low pitchmovement of the servo cylinder may be limited by engagernent of theappendageV 37 on the plate 86 with a low pitch stop 83 which will bedescribed in greater detail hereinafter.

ln' order to prevent the servo cylinder from engaging with the low pitchstop, it will be necessary to discontinue the" signal to theproportional solenoid and to reduce pressure (Ps) in the servo cylinder.The servo pressure may be reduced by imposing another signal upon theproportional solenoid, such signal being similar to the signaltransmitted when a propeller overspeeding condition arises. When suchsignal is received, the valve member 75 is elevated to Vpermit drain ofthe hydraulic fluid from the servo chamber through the control valve 46to the low pressure regulating valve 44. It will be quite apparent thatalternating signals can be transmitted in the proportional solenoid 'toreciprocate the servo cylinder 32 and thereby to* purge the system ofentrappcd air and supply all of the passages with hydraulic luid underpressure.

After the system has been supplied with hydraulic fluid, engineoperation may be instituted so that the main pump unit 10 Will functionand so that the auxiliary motor may be deenergized.V With the engine inoperation in accordance'wit-h a selected power setting, rtheproportional solenoid operated valve 46 will automatically control ow toand from the servo chamber to effect propeller speed control at thelevel in vkeeping with the selected power setting. This automaticoperation of the valve 46 was fully described above. Also during engineoperation, the topping governor will Ifunction to prevent dangerousoverspeeding of the propeller in the manner described.

As previously mentioned, the hydraulic system is utilized to causefeathering of the propeller and is also utilized to change'the pitchangle thereof to reverse positions. More specifically, when it isdesired that the propeller be feathered, the mechanical featheringplunger is thrust downwardly causing downward movement of the movablevalve member 9.1 in the shut oli valve 33. It will be noted that whenthe valve member 91 is thrust downwardly, the valve land V92 will closethe passage 34 tothe tlow ofl hydraulic uid at pump pressure. With thepassagev34 closed, there can be no high pressure tlowf `9 chamber suchas would cause low pitch movement of the servo cylinder. The movablevalve member 91 is provided with a downwardly extending appendage 93which engages a normally closed spring biased valve plug 94. Upon beingthrust downwardly, the valve member 91 thrusts the plug 94 downwardly topermit ow through the valve from a passage 95 to a passage 96. Thepassage 95 extends between the shut olf and feathering valve 33 and thetopping governor operated valve 45. At the valve 45, the passage 95communicates with a port which is normally open to ow from the passage62 which, as previously described, is connected to and contains fluid atthe pressure of the fluid within the servo chamber 42. Accordingly, whenthe shut ofI and featering valve is depressed, the servo chamber willdrain through the controlr valve 45 and through the shut olf valve intothe passage 96 which is connected to the pressurized sump. The drain ofhydraulic uid'from the servo chamber may be supplemented by drainthrough the proportional solenoid operated control valve 46. This maybe-eifected by providing means for transmitting an overspeeding signalto the proportional solenoid when the feathering valve is depressed.Upon transmission of such signal, the proportional solenoid will elevatethe valve member 75 to permit drain llow from the passage 62 to the lowpressure producing valve 44 in the manner previously described.

In all events, thedrain of the servo chamber 42 through the featheringand shut ol valve 33 will continue as long as the valve member 91 isdepressed. Accordingly, the servo cylinder 32 will shift toward fullhigh pitch causing the propeller to be feathered.

It has been mentioned that the solenoid operated valve 35 is energizedto effect reverse pitch movement of the propeller blades. It has alsobeen mentioned that a low pitch stop 88 is provided to limit the extentof normal low pitch movement. The arrangement is such that the low pitchstop is shifted from the path of movement of the servo cylinder when thevalve 35 is energized Ifor reverse pitch. The said stop is pivotallymounted at 100 within a cap-like member 101 which is threaded to theframe or block structure 102 surrounding the servo cylinder. It will beseen that by adjusting the cap-like member 101 the low pitch stop 88 canbe advanced toward or withdrawn from engagement with therservo cylinder.Itis important to note that the pivoted stop 88 is disposed within asleeve 103 which may be moved in a direction opposite to the servocylinder by hydraulic uid introduced through the passage 104. The sleeveengages a spring 105 which is seated against a lever 106 included in thepivoted stop structure and when the sleeve is shifted away from theservo cylinder by the force of hydraulic uid in the passage 104, thespring 105 will pivot the stop 88 in the clockwise direction, as viewedin the drawing, to permit the appendage 87 on the cylinder plate 86 topass the stop 88 as the cylinder is moved in the low pitch direction.After the cylinder appendage passes the low pitch stop, the propellerpitch angle will be changed to a reverse pitch. The limit of reversepitch movement of the propeller ymay' be fixed by a stop 107 which isadjustably secured within the cap-like member 101 to engage theappendage 87 in selectedpositions corresponding to selected reversepitch angles. y f

The passage 104 normally contains hydraulic fluid Vat atmosphericpressure, since said passage is connectedby a passage 108 whichcommunicates with the atmospheric sump 12 through the solenoidoperatedvalve 35. However, when the solenoid 109 included in the valve structureis energized, a valve member 110 is elevated to such a position that aland 111 closes the passage from the atmospheric sumpr and opens thepassage 8-to the tow of hydraulic fluid at pumppre'ssure from thepassage 34. Thus, when theV reversing solenoid 109 is, energized, thevalve 35 is positioned to cause hydraulic flow at pump pressure throughthe passage 104Y to jthesleeve 103 which pivots the low pitch stop outof position, permitting movement of the servo cylinder into reversepitch positions.

It will be noted that the passage 104 is also connected to a servo 112which includes a piston 113 adapted to engage a spring biased lever 114and pivot the same so that the said lever will engage or be in positionto engage the movable valve member in the topping governor valve 46 soas to prevent upward movement of said member 65. Accordingly, the servo112 is utilized to block the topping governor operated valve 45 andassure that the topping governor valve will not function to preventoverspeeding while the propeller blades are being set at reverse pitch.This prevents improper control 'operation by the topping governorcontrol valve as might occur while the propeller is being put in reversepitch positions.

The propeller pitch control and servo system as thus far described isthe same as the system shown in the Best application. As previouslymentioned, the system of this invention incorporates means for limitingthe flow rate of fluid to the servo motor so as to limit or reduce therate of decreasing the pitch angle as compared with the rate of increasethereof.

The said rate limiting means comprises a ow restrictor 200 located inthe high pressure passage 61 between the valves 45 and 46. The owrestrictor 200 is shown only schematically in Fig. l and may compriseany of a number of conventional elements adapted to cause a reduction inilow rate through a fluid conduit. It will be seen that the pressuredrop across the restrictor 200 will be substantially constant and lessthan 30 p.V s. i. With a substantially constant pressure drop, the flowrate can be limited to a substantially constant level for causing apitch change toward low pitch at a substantially constant andpreselected rate during normal pitch control operation. It will also beseen that the restrictor 200 has no etfect on flow rate, and thus noeifect upon high pitch change rate, when flow is from the servo chamber42. Thus, the rate of change toward high pitch will be substantiallygreater than change toward low pitch in normal operation. 'Ihe samelimited rate of change toward low pitch prevails when the propeller isbeing un-feathered or moved from the feathered angle. That is, inun-feathering, the hydraulic uid flows through the feathering valve 33,.the high pressure regulating valve 43, the control valve 45, and theconduit 61 containing the restrictor 200 before entering the controlvalve 46 and servo chamber 42.

Howeven it is desirable not to limit the ow and thus the pitch changerate when changing the pitch angle through the low pitch range into thereverse range. To provide for a rapid pitch change into the reversepitch range, a check valve 201 is disposed in a passageway 202 whichextends between conduit 104 and conduit 54. The check valve 201 may beof conventional form and is normally closed. When the reversing solenoid109 is energized to actuate valve 35 so as tov cause fluid at pumppressure to ow into conduit 104, the increased pressure opens the checkvalve 201 to the iiow of lluid from passageway 104 into passageway 54.Thus, the high pressure regulating valve 43 is effectively by-passed andfluid at pump pressure passes through the valve 45 to the restrictor200. Pump pressure. being substantially greater than the` pressurenormally in the line 61, the reduced pressure and llow rate'on thedownstream side of the restrictor 200 will be substantially greater andthe ow rate to the servo chamber 42 will be substantially, above thenormal level. Accordingly, the rate of reducing the pitch or blade anglewhen turning into the reverse range will be substantially greater thanthe normal rate of pitch reduction.

.It is the' primary purpose of providing the pressure regulating valves43 and 44 in the aforedescribed Vsystem to produce or provide a sourceof pressure a iixed amount greater and a fixed amount less than theservo Vpressure (Ps) in the servo chamber'42.V That'is, the valves 43and $44v at all timesproduce fluid pressures in the system ai,

accom-1;-

fixed;` arrrount` greater and` a; fixed amount. less than* the` thepressureregulating valves 43 and 44may be eliminated fromthe pitchcontrol and servo system where the servo i loads arev substantiallyconstant. Such` a system is disclosedin Fig. 2.

For the most part,v the` system-of Fig. 2 is similar to the` system ofFig,l; and; like elements of `thersystem are identified by thesamereferencenumerals. lnieliminating theipressure` regulating valves ,43and 44 from.fthe system ofFigfZ, thehigh pressure` lineV 54 to thegovernor valve 45 isidirectly connected to the passage 34;containingfluid at pumppressurefand the low pressuredine 58 to the` control valves45` and Y-46 is directly connected; to the drain line 60 extending tothe pressurized sump.; 15. In..addiV tion,the; servo chamber 42 is`connected by the` conduit 50 directly to the conduit` 63. With theaforedescribed modifications, the system of Fig. 2 is the same as thatshown in the previously mentioned :Best application `except; that the.high and low. pressure regulating valves are eliminated.

The system of Fig.` 2 isv further modified to embrace the improvementsof this invention, by providing a. re-

stricter 205 in the Aline 61 which` is the high pressure line betweenthe control valves 45 and 46., The restrictor 205 performs the samefunction.; as vthe restrictor 200 described in connection withtheembodiment? shown in Fig. 1. That is, the restrictor205l reduces theflow rate in the conduit-61 by providing a pressure drop from the pumppressure on the upstream side of the said restrictor. There isnorestrictor placed in the drain passage 58 or 58a so that iiuidfomaybedrained from the servo chamber 42'into the pressurized sump 15 withouta pressure drop and without flow, limitation,` other than the limitationimposed by valve 46.

Accordingly, in the embodiment `of the invention shown in Fig. 2, `theservo cylinder 32 `isrnoved in the-low. pitch direction by introducingfluid to thechamber 42 from tne pump, but the restrictor 205 reducestheY rate ofilow to the said servo chamber solas to. provide: for apreselected rate` of movementor rate of pitch change `inthe low pitch;direction. The servo `.cylinder 32'vistmoved in the high pitch directionby` draining uid from the servo chamber 42 into the pressurized` sump 15without limiting the flow so that movement ofxthe. servo` cylinder andpropeller in` the high pitch: direction will beata substantially greaterrate than the movement in` the low pitch direction. As in thefirstdescribed embodiment, the flow controlvalve 46 selectively connects theservo chamber 42 tothe high pressurelrsourcc and to the drain.

ln order that thereuvill; be. no undesirable 'ow rate4 limitationimposed when. the propeller is` being moved throughA the low pitch rangeinto the reverse pitch range, acheclcvalve 206 is provided inapassageway 207 interconnecting the high pressure port of the controlvalve t6 and `the conduit `104. As, in the lirstdescribed em.-bodirnentythe conduit 104 willcontain hydraulic fluid at pump pressurewhenf the solenoid operated `reversing valve 35` is energized. Wheny theconduit 104Hreceives iluid at pump pressure, the check valve-206` isopened tov permit the .bwof fluid.v at pump pressure in the` lineA 207to the control valve `46. lt is desirable to have some flowlimitation'inithe .line-.207andg forthis purpose, a iioiv restrictor,208i is provided in said line. Thev rea stricter 2.08 reduces` orlimits the` flow butt` to a lesser degree than` `the restrictor 205:.Accordingly,` the high, pressure flow, throughI the valve. 46. whenfthepropeller.l is.` beingr'moved :into .the` reverse pitch range bisgreater` u than` thehighpressureflow throughthe valve 4.6 duringA normalpitch control and` whenthe propeller` is being unfeatbered. Thus, the`rate of movement into low pitch' and'intoV the-reverse range Visgreater than the low pitch A changeratedduring normal operation. l

Accordingly, the, propeller pitch control and servo system of thisinvention as shown in the two described embodiments and in any otherembodiments thereof features means adaptedv to limit the rate of pitchchange inthe low pitch direction so as to reduce the possibility oipropeller overspeeding during normal pitch control operation and whenthepropeller is being` unfeathered. In addition, the invention embracesmeans for providing a more rapid rate of pitch change in the low pitchdirec- I tion when the propeller pitch is being reversed `through thelow pitch rangeinto the reverse pitch range.

The invention claimed is:

l. In an aeronautical propeller pitch control and servo system whichincludes an hydraulically operated servo motor` adapted to effect bladeangle changes toward high and low pitch in a range including thefeathered angle` and reversepitch of the propeller, the combinationcomprising a source of fluid under pressure, a drain, valve meansconnected with said servo motor and having a pair of passagewaysrespectively connected with thev source and withthe drain, the saidvalve means being operable to selectively connect the servo motor withsaid source or said drain either to increase or to decrease, the pitchangle, llow limiting means in. oneof said passageways to limit the rateof servo motor opcration when said servo motor is decreasing `the pitchangle during normal operation and when the propeller is being removedfrom the feathered angle,` and means for reversing the pitch. of thepropeller including a valve adapted to increase flow in said onepassageway whereby to increase the rate of servo motor operation indecreasing the` pitch angle. Y

2. In an aeronautical propeller pitch control and servo system whichincludes an hydraulically operated servo motor adapted'to effect bladeangle changes toward high and low pitch in, a range including thefeathered angle and reverse pitch ofthe propeller, the combinationcomprising a source of fluid under pressure, a drain, valve meansVconnected with said servo motor and having a high pressure side and alow pressureside, means connectingsaid high pressure side and saidsource, means` connectingsaid low pressure side and said` drain,saidlvalve means4 being operable to selectively connect said `servomotor to said source and to said' drain to either increase orto`decrease the pitch angle, flow limiting means in one of said connectingmeans, to limit the rateV of servo motor operation when said servo motoris decreasing the pitch .angle4 during normal operationand when thepropeller is being removed from the feathered angle, and means forreversing the pitch of the propeller including a valve adapted toincrease the ow into the highpressure side of said valve means wherebyto increase the rate of servo motor operation in decreasing the pitchangle.

3. In an' aeronautical propeller pitch control and servo system whichincludes an hydraulically` operated servo motor adapted to effect bladeangle changes within a rangeA whichincludes the feathered'angle andreverse pitch, the combination comprising a source of fluid underpressure, a drain, a propeller speed responsive valve having ahighpressure side connected with said source, a low pressure sideconnected with said` drain, and also having Vaconncction` with saidservo motor, said valve being constructed and arranged tointerconnectsaid l servo motor and said drainin'response to propellerover-y speedingand'to interconnect said source and saidservo motorinxresponse .to` propeller underspeedingand whenV the propelleris beingunfeathered, res'trictor means in the `connection between saidl sourceand saidivalve to reduce thev ow` offfluid from said source whereby to`limit the rate of blade angle lchange toward low pitch, and meansconnected with the highpressure side of said valve to increase thev flowthereto and to vsaid servo motorY when the propeller blade angleis beingreversed.Y

4. In an aeronautical propeller pitchcontrol and servo system whichincludes an hydraulically operated servo motor adapted to change thepropeller blade angle with said servo motor and being adapted toselectively v connect Vthe servo motor with the drain and with thesurce-to increase and decrease, respectively, the yblade angle, arestrictor in the connection between the source and the control valvefor providing a pressure drop therein whereby to control the flow ratetherethrough and to limit the rate of servo motor operation indecreasing the lblade angle, and control means for reversing the pitchof the propeller including a second valve having a conduit connectedwith the high pressure side or' the control valve and adapted toincrease the ow rate thereto whereby to increase the rate of servo motoroperation in decreasing the blade angle into reverse pitch.

5, In an aeronautical propeller pitch control and servo system whichincludes an hydraulically operated servo motor having a servo chamberand adapted to decrease and to increaseithe blade angle in response topressure increase 4and decrease, respectively, in the chamber, and tomake such blade angle changes within a range including a feathered angleand reverse pitch, the combination comprising a source of lluid underpressure, a drain, a control valve having a high pressure side and a lowpressure side respectively connected with said source and said drain,said valve being connected with said chamber and being adapted toselectively connect the chamber with they drain and withsaid source,` arestrictor in the connection between the source and the control valvefor providing a pressure drop therein whereby to control the flow ratetherethrough and to limit the rate of servo motor operation indecreasing theyblade angle, and control means for reversing the pitch ofthe propeller including a conduit connected with the high pressure sideof the control valve and Valso including a second valve in said conduitwhich is adapted to increase the flow rate tothe high pressure side ofthe said control valve whereby to increase the rate of servo motoroperation in decreasing the blade angle into reversepitch.

6. In an aeronautical propeller pitch control and servo system whichincludes an hydraulically operated servo motor adapted to decrease thepropeller-blade angle when it receives fluid under pressure anditoincrease theV blade angle when itY drains, the range of blade anglechanges including theV feathered angle and reverse pitch of thepropeller, the combination comprising a source of fluid under pressure,a drain, a ilow control valve having a high pressure side and a lowpressure side respectively connected with said source and said drain andhaving a connection with the servo motor to selectively connect the samewith the source and the drain, a restrictor in the connection betweenthe source and the valve to provide a pressure drop therein whereby tocontrol the ow rate therethrough and to limit the rate of servo motoroperation in decreasing the blade angle, and control means for reversingthe propeller pitch including a conduit connected with the high pressureside of the control valve, and a second valve in said conduit openingand closing said conduit to Huid under pressure whereby the ow to saidhigh pressure side may be increased to increase the rate of servo motoroperation in decreasf ing the blade angle into the reverse pitch range.

7. In an aeronautical propeller pitch control and servo system whichincludes an hydraulically operated servo motor having a servo chamberand adapted to respectively decrease and increase the propeller pitch inresponse to pressure increase and decrease in said chamber, thecombination comprising a source of fluid under pressure, a drain,pressure regulating means connected` with said source and responsive topressure in the chamber to provide hydraulic uid at a pressure a xedamount greater than fluid pressure in said chamber, a control valvehaving a high pressure side connected with said pressureregulating'means and a low pressure side connected with said drain, saidvalve being connected with said chamber and adapted to selectivelyconnect the same with the pressure regulating means and said drain, arestrictor in the connection between the valve and the pressureregulating means to reduce the flow rate therein whereby to limit therate of servo motor operation in decreasing the pitch, and control meansfor reversing the propeller pitch including means dening a passagewaybetween said source and said connection, and a normally closed valve insaid passageway which when opened introduces Huid at the pressure ofsaid source to said restrictor whereby Vthe flow rate in said connectionis increased to increase the rate of se-rvo motor operation indecreasing the blade angle into reverse pitch.

8. In an aeronautical'propeller pitch control and servo system whichincludes an hydraulically operated servo motor having a servo chamberand adapted to decrease and to increase the blade angle in response topressure increase and decrease, respectively, within said chamber,

the combination comprising a source of iiuid under pressure, a drain,rst pressurerregulating means Vconnected with said source and responsiveto uid pressure in said chamber to providehydraulic uid at a pressure afixed amount greater than pressure in said chamber, second pressureregulating means connected with said drain and responsive to iluidpressure in said chamber to provide a fluid pressure a iixed amount lessthan pressure in said` chamber which fixed-amount is substantially equalto the iirst mentioned fixed amount, a control valve connected with saidchamber and having aV high pressure side and alow pressure sideconnected with said iirst and second pressure regulating means,respectively, said valve being adapted to selectively fconnect saidchamber with said tirst and second pressure regulatingmeans, arestrictor disposed in the Aconnection between said high pressure sideand said first pressure regulating means for providinga .pressuredroptherein Wherebyto limit flow therethrough and to limit servorrnotoroperation in decreasing the blade anglel to a rate less'than therate ofoperation in increasingthe blade` angle, and control means for reversingthe propeller pitch including means dening a passageway between saidsource and said connection, and Y a normally closed valve `in saidpassageway which when opened introduces uid at the pressure of saidsource to said restrictor whereby the flow rate in said connection isincreased to increase the rate of servo motor operation when decreasingthe blade angle into the reverse pitch range. r

9. In an aeronautical propeller pitch control and servo system whichincludes an hydraulically operatedservo motor having a servo chamber andadapted to respectively decrease and increase the propeller blade anglein response to pressure increase and decrease in said chamber, thecombination comprising a source of uid under pressure, a drain, pressureregulating means connected with said source and responsive to pressurein the charnber to provide hydraulic fluid at a pressure a iixed amountpressure-regulatngameans to reducetthe flow rate therein whereby toflirnitthe nate/of servo motor operation, in decreasing thebladesangle,andvcontrol means for4 reversing the propellerpitchpineludingmeansdening a passageway communicating -with said, connection, a checkvalve in said passagewaypreventing ilow from saidconneetion through saidpassageway, and a control` valve for connecting,saidpassageway and `said`source to introduce fluid at' the pressure 01E- said source to` said`restrictor whereby the ilow rate insaid connection is increased toincrease the rate ofservo motor operation when decreasing thebladefangleintoreverse pitch, p

10; In `an aeronautical,` propeller pitch control and servo system whichincludes an hydraulically operated servo motor adapted,tofincreaseganddecrease propeller pitch Lin` ,a range: includingi a feathered angleandreverse pitch, the combination comprising a source of uid underpressure, a-drain, a controlvalvehaving a high pressure side connectedwith the source and a'low` pressure side connectedwith `the drain saidvalve being adapted to selectively connect its highpressureandrlowpressure sides withi the servo motor to respectivelydecrease and increasethe` pitch, a owV restrictor in the ,connectionbetween said valve andsaidrsource tollimit the rate of` servo motoroperation in decreasing the pitch, `and control` means for reversing`thel pitch including means defining a passageway, between said sourceand the high presf sure side `of said controlvalve, and' a` normallyclosed valve insaid passageway which when opened provides for an`increased rate of servo motor operation in dccreasing the pitch.

l1. In` an aeronautical.' propeller pitch control andy servo system'which includesfanhydraulically `operated servo motor adapted to`increase rand to decreasepropeller pitch, the combination comprisinglasource` of lluid under pressure, a drain,- a controlV valvefconnected tothe servo motor and having ,alA highpressure side connected with thesource anda low pressure side `connected with` the drain, said4 valvebeing operable to Vselectively connect the servo motor with `its highpressure side` and low pressure side` to'rsspectively' decreaseandincrease the pitch, a ow restrictor. in the connection` between said,

valve `and said source to limitth'e rate of servo motor operation indecreasing `the pitch, land `control means forv reversing thepitch'inclridingA means defining` asecond passagewayl between saidsourceandthe high pressure side of-said control valve, a second control`valveA in; y

said second passageway' which is normally opelilfto..l atmosphere andnormallywclosedo Asaid source, ,anda

checkvalve in said secondjpassagewayfpreventingfowfrom saidihlghpressure side Ato saidsecond' valve, saidA secondfvalve being adapted to:selectively permitow frorngsaidsource in saidgsecond passageway tosaidhighl'pressurc side of saidcontrol valve to( provide for an increasedrate of servo-motor operation in decreasing the pitch.`

12.' In, an aeronautical propellerpitchl controly andAV servo motoradapted to increasel and. decrease the,

featheredangle Vand `reverse pitch, the combination com-` prisingAa-source` of uid under pressure, adrain, a control valve connected tosaid servo?. motor andhaving a high pressureprside` connected with thesourceand a low pressure side connectede with` the drain, said` valvebeing adapted to selectively interconnectthe servomotor with said highand*` low` pressure sides to respectively decrease and increasethepitch; a flow Vrestrictor in the connection betweensaid valve andA saidsource to `limit the ratev of servomotor operation` indecreasing the`pitch, and control `means for reversing the pitch including meansvdefiningarsecond-passageway,between said source and the Yhigh pressureside of said control valve, a normally closedvalve in said secondpassageway, and'asecond restrictor in said. second passagewaypermitting,A ow therethrough suilicient to. ,increase ,the rate of servomotor operationein. decreasing the pitch.` p p 13. In an aeronauticalpropeller pitch control and servo` system` which includes, an`hydraulically operated servomotor adaptedrto)` increase andrdecreasepropeller pitch in arange including the feathered angle and reversepitch, thefcornbination` comprisinga source of fluid vunder pressure,amdrain,` acontrol valve connected to said'servo moton and4 having ahigh pressure side connected with the source-and1 aA low pressure sideconnected withthe drainrsaidvalve being'radapted to selectively` connectthe servovmotoi: with its Yhigh and low pressure sides `to, respectivelydecrease, and increase the pitch; a ow restrictor in` the connectionbetween said valve and said source to limit-the rate` of servo motoroperation in decreasing the` pitch, anda control meansfor reversing, thepitchfincludingmeans: defining a s'econdpassagewaybetween saidsource andthe high pressure side'f'of` the said control valve, aL second valve insaid secondV passageway whichis normally` closed to said sourceand ropentoatmosphere, a check valve in said second pssag'ewa'ypreventing tlowfrom said high pressure side to said, second valve,v a `secondrestrictor in said"passageway, and [said second valve. being adapted topermit flow between saidsource and said high pressure side through-saidsecond` restrictor at a rate providing forlan increased rate ofservdmotor operation in decreasing the pitch.

ReferencesCit/edin.theiile of `this patent UNITED STATES: PATENTS'

